Dichlorophenols are useful intermediates in the preparation of a variety of chemical products. In particular, certain herbicides are prepared from dichlorophenols. For example, 2,4-dichlorophenoxyacetic acid (2,4-D) can be prepared from 2,4-dichlorophenol. See U.S. Pat. Nos. 2,480,817 and 2,651,659. Also, 3,6-dichloro-2-methoxybenzoic acid (dicamba) can be prepared from 2,5-dichlorophenol. See, for example, U.S. Pat. Nos. 3,013,054 and 5,648,562.
With the introduction of dicamba-tolerant traits in various crop plants, dicamba is becoming an increasingly important herbicide. However, difficulties remain in processes for producing dicamba. High raw material cost, low process conversions and selectivities, and large amounts of wastes are problems that remain in these processes. In addition, the starting material 2,5-dichlorophenol is expected to significantly contribute to high raw material costs. Accordingly, there remains a need for improved processes for the production of critical intermediates to dicamba including 2,5-dichlorophenol to improve process economics.
U.S. Pat. No. 4,568,777 proposes a process for the isomerization of mono- or dichlorophenols, including 2,4-dichlorophenol, using certain types of zeolite catalysts. However, this process failed to achieve a high selectivity for the 2,5-dichlorophenol isomer. Thus, there remains a need for commercially feasible processes that are highly selective for 2,5-dichlorophenol, which reduce raw material costs and waste products.